Radio communication system, radio station, radio terminal, communication control method, and non-transitory computer readable medium

ABSTRACT

A radio terminal (3) can perform carrier aggregation using a first cell (10) of a first radio station (1) and a second cell (20) of a second radio station (2). The first radio station (1) or the second radio station (2) transmits constraint information to the radio terminal (3). The constraint information contains an information element necessary to specify a reception constraint and/or transmission constraint related to the first cell (10) and/or the second cell (20) when the carrier aggregation is performed. The reception/transmission constraint is a constraint related to downlink signal reception/uplink signal transmission by the radio terminal over one or more subframe periods of the primary cell (10) and the secondary cell (20). It is thus, for example, possible to contribute to reduction in wasteful power consumption in the radio terminal in the carrier aggregation of a plurality of cells served by different radio stations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a continuation patent application of U.S.patent application Ser. No. 14/768,088, filed Aug. 14, 2015, which is anational stage application of International Application No.PCT/JP2013/007094 entitled “Radio Communication System, Radio Station,Radio Terminal, Communication Control Method, and Non-TransitoryComputer Readable Medium,” filed on Dec. 3, 2013, which claims thebenefit of the priority of Japanese Application No. 2013-033703 filed onFeb. 22, 2013, the disclosures of each of which are hereby incorporatedby reference in their entirety.

TECHNICAL FIELD

The present invention relates to a radio communication system in which aradio station communicates with a radio terminal by using a plurality ofcells.

BACKGROUND ART

In order to improve the deterioration of communication quality due todrastic increases in mobile traffic in recent years and achieve fastercommunication, the standardization of Carrier Aggregation (CA) functionsthat enable a radio terminal (User Equipment (UE)) to communicate with aradio base station (eNode B (eNB)) by using a plurality of cells hasbeen undertaken in the 3GPP Long Term Evolution (LTE). Note that thecells that a UE (User Equipment) can use in CA are limited to aplurality of cells of one eNB (i.e., a plurality of cells served by oneeNB).

The cells that are used by a UE in CA are categorized into a PrimaryCell (PCell) that has already been used as a serving cell when the CA isstarted and a Secondary Cell(s) (SCell(s)) that is used in addition tothe PCell or in dependence thereon. Each SCell can be used by a UE asthe need arises, and the use of them can be stopped. Note that startingthe use of an SCell is called “activating” or “activation”. Similarly,stopping the use of an SCell is called “deactivating” or “deactivation”.Non-Access Stratum (NAS) mobility information, security information(security input) and the like are transmitted and received through aPCell during radio connection establishment (RRC connectionEstablishment/Re-establishment) (see Non-patent Literature 1). Adownlink (DL) Carrier and an uplink (UL) Carrier corresponding to aPCell are called “DL Primary Component Carrier (PCC)” and “UL PCC”,respectively. Similarly, a DL Carrier and a UL Carrier corresponding toa SCell are called “DL Secondary Component Carrier (SCC)” and “UL SCC”,respectively.

A downlink data (DL data) transmission operation in CA is explained withreference to FIG. 17 (Non-patent Literature 2). Here, it is assumed thata UE uses a first cell (Cell1) and a second cell (Cell2) served by aneNB as a PCell and an SCell, respectively. In a step S1, the eNBtransmits, to the UE, configuration information for the SCell (i.e., theCell2) (RRC Connection Reconfiguration (SCell configuration)). In a stepS2, the eNB transmits to the UE an instruction indicating the activationof the Cell2 (Activation control element (activation of SCell)). In astep S3, the UE starts to use the SCell (SCell activation). In steps S4and S5, the eNB transmits DL data to the UE by using the PCell and theSCell.

In a step S6, the eNB determines that it no longer needs to use theSCell for the UE and hence transmits an instruction indicating thedeactivation of the SCell (Deactivation control element (deactivation ofSCell)). In a step S7, the UE suspends the use of the Cell2 (SCelldeactivation). In a step S8, the eNB and the UE transmit/receive DL databy using only the PCell.

In a step S9, the eNB determines that it needs to use the SCell for theUE again and hence transmits an instruction indicating the activation ofthe SCell (Activation control element (activation of SCell)). In a stepS10, the UE starts to use the SCell (SCell activation). In steps S11 andS12, the eNB transmits DL data to the UE by using the PCell and theSCell.

As explained above, the eNB can control whether the SCell should be used(activated) or not according to the data amount (also called “trafficamount”) regarding the UE. This makes it possible to improve thethroughput for each UE while avoiding the increase in the powerconsumption which would be otherwise caused by the unnecessary decodingof DL control signals (Physical Downlink Control Channel: PDCCH)performed by the UE.

CITATION LIST Non Patent Literature

Non-patent Literature 1: 3GPP IS 36.300 V 11.3.0, “Evolved UniversalTerrestrial Radio Access (E-UTRA) and Evolved Universal TerrestrialRadio Access Network (E-UTRAN); Overall description; Stage 2 (Release11)”, Section 7.5, September 2012 Non-patent Literature 2: 3GPP TS36.321 V 11.0.0, “Evolved Universal Terrestrial Radio Access (E-UTRA);Medium Access Control (MAC) protocol specification (Release 11)”,Section 6.1.3.8, September 2012 Non-patent Literature 3: 3GPPRWS-120046, Samsung Electronics, “Technologies for Rel-12 and Onwards”,3GPP TSG RAN Workshop on Rel-12 and Onwards, Ljubljana, Slovenia, 11-12Jun. 2012 Non-patent Literature 4: 3GPP RWS-120010, NTT DOCOMO,“Requirements, Candidate Solutions & Technology Roadmap for LTE Rel-12Onward”, 3GPP TSG RAN Workshop on Rel-12 and Onwards, Ljubljana,Slovenia, 11-12 Jun. 2012

SUMMARY OF INVENTION Technical Problem

Further, inter-base station carrier aggregation (inter-eNB CA) in whichcells of different radio base stations (eNBs) are simultaneously usedhas been proposed (Non-patent Literatures 3 and 4). For example, a cellof a macro base station (Macro eNB (MeNB)) and a cell of a low-powerbase station (Low Power Node (LPN)) are simultaneously used as a PCelland an SCell, respectively. In inter-base station (or inter-eNB) carrieraggregation, bearers are independently configured in the PCell and theSCell and communication is performed between an UE and the MeNB andbetween the UE and the LPN.

For example, a radio terminal (UE) performs voice communication (called“Voice over IP (VoIP)” or “Voice over LTE (VoLTE)”) in the PCell andperforms data communication (e.g., FTP) in the SCell. In general, VoIPcommunication is not performed often. Meanwhile, FTP traffic changesaccording to the user activities. In the case of the ordinary CA wherecells of the same base station are used, the eNB can adaptively controlthe activation/deactivation of the SCell according to the FTP traffic.However, in the case where the PCell and the SCell are served bydifferent eNBs, it is difficult for the eNB (e.g., an MeNB) that servesthe PCell to adaptively control the activation/deactivation of the SCellserved by the other eNB (e.g., an LPN) according to the FTP trafficprovided in that SCell. Accordingly, for example, the UE has tounnecessarily decode a downlink control signal (PDCCH) in the SCellwhere communication is not performed often, thus causing a possibilitythat the electric power is wastefully consumed. This could cause aproblem especially in the case where the resource scheduling of thePCell and that of the SCell are independently performed in differentbase stations. This is because when each radio base stationindependently performs scheduling, it is possibly difficult to use thePDCCH of the PCC for the scheduling of data transmission performed inthe SCC (i.e., to perform the so-called cross-carrier scheduling).

Further, in the carrier aggregation, the SCell is activated on theprecondition that the UE has an active connection with the PCell. Inother words, the SCell is additionally or dependently activated on thecondition that the UE is in connection with the PCell. Therefore, the UEcannot deactivate the PCell while maintaining the SCell in the activatedstate. For this reason, the UE has to maintain a state that the UE canreceive or transmit on the PCell even when VoIP communication in thePCell is rarely performed.

One of the objects of the present invention is to provide a radiocommunication system, a radio station, a radio terminal (LIE), acommunication control method, and a program which are contribute toreduction in wasteful power consumption of a radio terminal (UE) incarrier aggregation of a plurality of cells served by different radiostations.

Solution to Problem

In a first aspect, a radio communication system includes a first radiostation that serves a first cell, a second radio station that serves asecond cell, and a radio terminal capable of performing carrieraggregation using the first and second cells. Further, the first orsecond radio station is configured to transmit constraint information tothe radio terminal. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In a second aspect, a first radio station that serves a first cellincludes a communication control unit. The communication control unitsupports carrier aggregation using the first cell and a second cellserved by a second radio station. Further, the communication controlunit transmits constraint information to a radio terminal that performsthe carrier aggregation. The constraint information contains aninformation element necessary to specify at least one of a receptionconstraint and transmission constraint related to at least one of thefirst and second cells when the carrier aggregation is performed. Thereception constraint is a constraint related to downlink signalreception by the radio terminal over one or more subframe periods of thefirst and second cells. The transmission constraint is a constraintrelated to uplink signal transmission by the radio terminal over the oneor more subframe periods.

In a third aspect, a second radio station that serves a second cellincludes a communication control unit. The communication control unitsupports carrier aggregation using a first cell served by a first radiostation and the second cell. Further, the communication control unittransmits constraint information to a radio terminal that performs thecarrier aggregation. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In a fourth aspect, a radio terminal includes a communication controlunit that supports carrier aggregation using a first cell served by afirst radio station as a first cell and using a second cell served by asecond radio station as a second cell. Further, the communicationcontrol unit receives constraint information from the first or secondradio station. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In a fifth aspect, a communication control method performed in a firstradio station that serves a first cell includes transmitting constraintinformation to a radio terminal that performs carrier aggregation. Thecarrier aggregation uses the first cell and a second cell served by asecond radio station. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In a sixth aspect, a communication control method performed in a secondradio station that serves a second cell includes transmitting constraintinformation to a radio terminal that performs carrier aggregation. Thecarrier aggregation uses a first cell served by a first radio stationand the second cell. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In a seventh aspect, a communication control method performed in a radioterminal includes performing carrier aggregation using a first cellserved by a first radio station and a second cell served by a secondradio station, and receiving constraint information from the first orsecond radio station. The constraint information contains an informationelement necessary to specify at least one of a reception constraint andtransmission constraint related to at least one of the first and secondcells when the carrier aggregation is performed. The receptionconstraint is a constraint related to downlink signal reception by theradio terminal over one or more subframe periods of the first and secondcells. The transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over the one or more subframeperiods.

In an eighth aspect, a program includes instructions for causing acomputer to perform a communication control method according to theabove-described fifth aspect.

In a ninth aspect, a program includes instructions for causing acomputer to perform a communication control method according to theabove-described seventh aspect.

In a tenth aspect, a program includes instructions for causing acomputer to perform a communication control method according to theabove-described seventh aspect.

Advantageous Effects of Invention

According to the above-described aspects, it is possible to provide aradio communication system, a radio station, a radio terminal (UE), acommunication control method, and a program which are contribute toreduction in wasteful power consumption of a radio terminal (UE) incarrier aggregation of a plurality of cells served by different radiostations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a configuration example of a radio communication systemaccording to a first embodiment;

FIG. 2 shows a configuration example of a first radio station accordingto the first embodiment;

FIG. 3 shows a configuration example of a second radio station accordingto the first embodiment;

FIG. 4 shows a configuration example of a radio terminal according tothe first embodiment;

FIG. 5 is a sequence diagram showing an example of a communicationcontrol method in a radio communication system according to the firstembodiment (Procedure Example 1);

FIG. 6 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the firstembodiment (Procedure Example 2);

FIG. 7 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the firstembodiment (Procedure Example 3);

FIG. 8 is a sequence diagram showing an example of a communicationcontrol method in a radio communication system according to a secondembodiment (Procedure Example 4);

FIG. 9 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the secondembodiment (modification of Procedure Example 4);

FIG. 10 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the secondembodiment (Procedure Example 5);

FIG. 11 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the secondembodiment (Procedure Example 6);

FIG. 12 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the secondembodiment (Modification of Procedure Example 6);

FIG. 13A is a sequence diagram showing another example of acommunication control method in a radio communication system accordingto the second embodiment (Option 1 of Procedure Example 7);

FIG. 13B is a sequence diagram showing a receiving period (ON-period)and a non-receiving period (OFF-period) in the option 1 of ProcedureExample 7;

FIG. 14A is a sequence diagram showing another example of acommunication control method in a radio communication system accordingto the second embodiment (Option 2 of Procedure Example 7);

FIG. 14B shows a receiving period (ON-period) and a non-receiving period(OFF-period) in the option 2 of Procedure Example 7;

FIG. 15A is a sequence diagram showing another example of acommunication control method in a radio communication system accordingto the second embodiment (Option 3 of Procedure Example 7);

FIG. 15B shows a receiving period (ON-period) and a non-receiving period(OFF-period) in the option 3 of Procedure Example 7;

FIG. 16 is a sequence diagram showing another example of a communicationcontrol method in a radio communication system according to the secondembodiment (Option 1 of Procedure Example 7); and

FIG. 17 is a sequence diagram showing a carrier aggregation procedureaccording to the LTE (Background Art).

DESCRIPTION OF EMBODIMENTS

Specific embodiments are explained hereinafter in detail with referenceto the drawings. The same symbols are assigned to the same orcorresponding elements throughout the drawings, and duplicatedexplanations are omitted as necessary.

First Embodiment

FIG. 1 shows a configuration example of a radio communication systemaccording to this embodiment. The radio communication system accordingto this embodiment includes a first radio station 1, a second radiostation 2, and a radio terminal 3. The radio stations 1 and 2 areconnected to a core network 4 and serve first and second cells 10 and20, respectively. Each of the radio stations 1 and 2 is, for example, aradio base station, a base station control station, or a simplifiedradio base station having only some of the functions (protocol layers)of an ordinary radio base station. The radio terminal 3 has a functionof, while using a cell of one radio base station, using a cell ofanother radio station. In other words, the radio terminal 3 supports acarrier aggregation (or cell aggregation) of a plurality of cells servedby different radio stations. Note that the different radio stations maybe different base stations independent of each other, or may be oneradio station and another radio base station dependent on the one radiostation. Further, the different radio stations may be radio stations ofdifferent types having different functions.

For example, the radio terminal 3 can establish a second radioconnection on the second cell 20 while maintaining a first radioconnection on the first cell 10. In this way, the radio terminal 3 cansimultaneously use a plurality of cells (e.g., the cells 10 and 20) fortransmitting or receiving signals (e.g., user data or controlinformation). Note that, the expression “simultaneous use of a pluralityof cells” is not limited to actual simultaneous reception ortransmission of signals in a plurality of cells. That is, it includes: astate where the radio terminal actually receives or transmits signals ineither one of the cells although the radio terminal is able to receiveor transmit signals in both of the cells; a state where the radioterminal receives or transmits signals of different types in therespective cells; and a state where the radio terminal uses each of theplurality of cells for either signal reception or signal transmission.

In view of the carrier aggregation of a plurality of cells served bydifferent radio stations, the function of using a plurality of cellsserved by different radio stations can be called “inter-radio stationcarrier aggregation”. Further, in view of the above-describedsimultaneous use of a plurality of cells, the function of using aplurality of cells served by different radio stations can also be called“Dual Connection”, “Dual Connectivity”, “Multi Connection”, “MultiConnectivity”, or the like.

The radio terminal 3 may transmit to the radio station 1 or the radiostation 2 a terminal capability report indicating that the radioterminal 3 is capable of performing inter-radio station carrieraggregation (i.e., supports inter-radio station carrier aggregation).Alternatively, the radio terminal 3 may implicitly indicate that theradio terminal 3 supports inter-radio station carrier aggregation by thecategory of the radio terminal 3 or its device release number. Thecapability of performing inter-radio station carrier aggregation canalso be called “dual-connection capability” or “multi-connectioncapability”.

FIG. 1 shows a Heterogeneous Network (HetNet) environment. Specifically,the first cell 10 shown in FIG. 1 has coverage wider than that of thesecond cell 20. Further, FIG. 1 shows a hierarchical cell structure inwhich the second cell 20 is disposed inside the first cell 10. Note thatthe cell structure shown in FIG. 1 is merely an example. For example,the first and second cells 10 and 20 may have the same degree ofcoverage. In other words, the radio communication system according tothis embodiment may be applied to a Homogeneous Network environment.

Next, an operation of the radio communication system according to thisembodiment is explained in a more detailed manner. The radiocommunication system according to this embodiment enables the radioterminal 3 to discontinuously perform reception or transmission at aninterval equal to or longer than a signal transmission/reception unitperiod (e.g., one subframe) in at least one of the first and secondcells 10 and 20 when the inter-radio station carrier aggregation hasbeen configured. The time in which the inter-radio station carrieraggregation has been configured can be expressed in different words as atime in which the radio terminal 3 is instructed to use the second cell20 when the radio terminal 3 is already using the first cell 10.Further, the time in which the inter-radio station carrier aggregationhas been configured can also be expressed as a time in which the radioterminal uses a Component Carrier (CC) (e.g., at least downlink CC) ofthe first radio station 1 as a Primary-Component Carrier (PCC) and alsouses a Component Carrier (CC) (e.g., at least downlink CC) of the secondradio station 2 as a Secondary-Component Carrier (SCC). When theinter-radio station carrier aggregation has been configured, the radioterminal 3 has active connections for both the first and second cells 10and 20.

To enable the radio terminal 3 to perform discontinuous reception ortransmission when the inter-radio station carrier aggregation has beenconfigured, the radio communication system according to this embodimentoperates as follows. That is, the first radio station 1 or 2 transmitsconstraint information to the radio terminal 3. The constraintinformation contains an information element necessary to specify atleast one of a reception constraint and transmission constraint for atleast one of the first and second cells 10 and 20 when the inter-radiostation carrier aggregation is performed. Note that the receptionconstraint is a constraint related to downlink signal reception by theradio terminal 3 over one or more signal transmission/reception unitperiods (e.g., one or more subframes). For example, the receptionconstraint includes a constraint indicating that the radio terminal 3does not have to receive or decode a predetermined downlink signal in apredetermined period (or at a predetermined timing), a constraintindicating that the radio terminal 3 should receive or decode apredetermined downlink signal in a predetermined period (or at apredetermined timing), or the like. Examples of the predetermineddownlink signal include a signal that is transmitted through a downlinkcontrol channel (e.g., the PDCCH in the LTE), a signal for transmittingpaging, and a signal for transmitting system information. Thetransmission constraint is a constraint related to uplink signaltransmission by the radio terminal 3 over one or more signaltransmission/reception unit periods (e.g., one or more subframes). Forexample, the transmission constraint includes a constraint indicatingthat the transmission of an uplink signal (e.g., the Physical UplinkShared Channel (PUSCH) in the LTE) by the radio terminal 3 is prohibitedin a predetermined period (or at a predetermined timing), a constraintindicating that the transmission of an uplink signal is possiblypermitted in a predetermined period (or at a predetermined timing), orthe like. The one subframe period means the Transmission Time Interval(TTI) specified in the communication standards to which the radiocommunication system conforms (e.g., the 3GPP LTE). The length of onesubframe period in the LTE is one millisecond.

For example, in order to specify at least one of the receptionconstraint and the transmission constraint, the constraint informationmay explicitly or implicitly indicate at least one of the followingitems (a) to (h):

(a) A period (or timing) in/at which a predetermined downlink signalshould be received;

(b) A period (or timing) in/at which a predetermined downlink signalshould be decoded;

(c) A period (or timing) in/at which a predetermined downlink signaldoes not have to be received;

(d) A period (or timing) in/at which a predetermined downlink signaldoes not have to be decoded;

(e) A period (or timing) in/at which the transmission of an uplinksignal is prohibited;

(f) A period (or timing) in/at which an uplink signal can betransmitted;

(g) A period (or timing) in/at which an uplink signal may betransmitted; and

(h) A period (or timing) in/at which the transmission of an uplinksignal may be permitted.

Although the above-described periods or timings are expressed from thestandpoint of the radio terminal 3, they have the same meanings evenwhen they are expressed from the standpoint of the radio stations 1 and2.

Further, examples of possible reception constraints also include asetting of “reception gap” indicating the above-described item (c) or(d). Alternatively, it is also conceivable to indicate “DiscontinuousReception (DRX)” that is specified by a combination of theabove-described items (a) and (c) or items (b) and (d). Note that aperiod corresponding to the item (a) or (b) is called “receptionperiod”, “reception On-period (or On-period)”, or “Active period”.Further, a period corresponding to the item (c) or (d) is called“non-reception period”, “reception Off-period (or Off-period)”, or“Inactive period”.

As an example of the transmission constraint, it is conceivable to set“transmission gap” indicating the above-described item (e).Alternatively, it is also conceivable to indicate “masking oftransmission timing” that is specified by a combination of the item (e)and one of the items (f) to (h), i.e., narrows down the timing (e.g.,the subframe) at which the transmission of an uplink signal ispermitted. Further, it is also conceivable to indicate “DiscontinuousTransmission (DTX)” that is specified by a combination of the item (e)and one of the items (f) to (h). Note that a period corresponding to theitem (e) is called “non-transmission period”, “transmission Off-period(or Off-period)”, or “Inactive period”. Further, a period correspondingto the item (f), (g) or (h) is called “transmission period”,“transmission On-period (or On-period)”, or “Active period”

Further, the constraint information may directly or implicitly indicateat least one of the reception constraint and the transmissionconstraint. For example, the constraint information may include at leastone of the following items:

(A) A request (or instruction) for application of a receptionconstraint;

(B) A request (or instruction) for application of a transmissionconstraint;

(C) Contents of a reception constraint;

(D) Contents of a transmission constraint;

(E) Control parameters regarding a reception constraint; and

(F) Control parameters regarding a transmission constraint.

The item (A) is used, for example, when the application of a receptionconstraint is requested in a state where the radio stations 1 and 2, orthe radio stations 1 and 2 and the radio terminal 3, have commoninformation indicating what kind of constraint is indicated by thereception constraint, or that information has been defined in thespecifications. Similarly, the item (B) is used, for example, when theapplication of a transmission constraint is requested in a state wherethe radio stations 1 and 2 have common information indicating what kindof constraint is indicated by the transmission constraint, or thatinformation has been defined in the specifications.

The item (C) is information indicating what kind of reception constraintis applied. For example, the item (C) indicates the setting of“reception gap”, the instruction of “DRY”, or the like. Similarly, theitem (D) is information indicating what kind of transmission constraintis applied. For example, the item (D) indicates the setting of“transmission gap”, the instruction for “masking of transmissiontiming”, or the like.

The item (E) is information indicating a control parameter(s) thatshould be set in the reception constraint. For example, the item (E)indicates at least one of the above-described items (a) to (d).Similarly, the item (F) is information indicating a control parameter(s)that should be set in the transmission constraint. For example, the item(F) indicates at least one of the above-described items (e) to (h).

The radio terminal 3 may operate, for example, as follows. The radioterminal 3 receives constraint information from the radio station 1 or2. Then, the radio terminal 3 performs a receiving operation inaccordance with a reception constraint or a transmitting operation inaccordance with a transmission constraint, in the inter-radio stationcarrier aggregation. For example, the radio terminal 3 may stop thedecoding of the PDCCH of the second cell (i.e., the PDCCHblind-decoding) in one or more subframes included in a receptionconstraint for the second cell specified based on a reception constraintincluded in the constraint information. In this case, the receptionconstraint indicates that the radio terminal 3 does not have to decode apredetermined downlink signal in/at a predetermined period (or timing).Further, the radio terminal 3 may stop the operation of a transmissioncircuit components (e.g., an amplifier) related to the second cell orchange the operation mode of the transmission circuit components to alow-power mode in one or more subframes included in a transmissionconstraint for the second cell specified based on a transmissionconstraint included in the constraint information. In this case, thetransmission constraint indicates that the transmission of an uplinksignal (e.g., the Physical Uplink Shared Channel (PUSCH) of the LIE) bythe radio terminal 3 is prohibited in/at a predetermined period (ortiming). In this way, the radio terminal 3 can suspend the receivingoperation or the transmitting operation based on the constraintinformation when the inter-radio station carrier aggregation is beingperformed.

The constraint information may be applied to both of the first andsecond cells 10 and 20, or may be applied to either one of them. Forexample, the radio terminal 3 may derive the reception constraint or thetransmission constraint (e.g., a reception gap, a transmission gap, areception period, or a transmission period) for the second cell 20 byusing the reception constraint or the transmission constraint (e.g., areception gap, a transmission gap, DRX, or masking of transmissiontiming) for the first cell 10. In this case, the constraint informationis only required to include an information element(s) indicating thereception constraint or the transmission constraint for the first cell10 in order to specify the reception constraint or the transmissionconstraint for the second cell 20.

The reception constraint (or the transmission constraint) for the secondcell 20 based on the constraint information may be configuredindependently of that for the first cell 10. An advantage of this methodis that it is unnecessary to take account of the states of services(i.e., data transmission/reception) performed for the same radioterminal 3 in the radio stations 1 and 2.

Alternatively, the reception constraint (or transmission constraint) ofthe second cell 20 may be configured so as to be dependent on that forthe first cell 10. For example, the reception gap (or transmission gap)in the second cell 20 may be configured so that at least a part of thereception gap (or transmission gap) in the second cell 20 does notoverlap that in the first cell 10. For example, the reception gaps inthe first and second cells 10 and 20 may be configured so that the startpoint of the reception period in the second cell 20 does not coincidewith the start point of the reception period in the first cell 10.Similarly, the transmission gaps in the first and second cells 10 and 20may be configured so that the start point of the transmission period inthe second cell 20 does not coincide with the start point of thetransmission period in the first cell 10. An advantage of this method isthat it is possible to reduce the load on the radio terminal 3 andreduce the delay in its data transmission/reception by defining a periodin which the radio terminal 3 does not need to simultaneously receivedownlink signals from the radio stations 1 and 2 at the start of areception period or defining a period in which the radio terminal 3 doesnot need to simultaneously transmit uplink signals to the radio stations1 and 2 at the start of a transmission period, thereby enablingtransmission/reception of as much downlink data or uplink data aspossible in that period.

Further, for example, the reception gaps (or transmission gaps) in thefirst and second cells 10 and 20 may be determined so that the receptionperiod (or transmission period) in the first cell 10 does not overlapthat in the second cell 20 at all. An advantage of this method is thatit is possible to prevent the radio terminal 3 from simultaneouslyreceiving downlink signals from (or simultaneously transmitting uplinksignals to) the radio stations 1 and 2 and thereby to considerablyreduce the load on the radio terminal 3. In particular, when the radioterminal 3 is using a plurality of cells operated in different frequencyhands, the load can be reduced. This is because it is conceivable toequip the radio terminal 3 with a hardware component (e.g., a radiofrequency (RF) unit) for each frequency band in order to use a pluralityof cells using different frequency bands, and it is expected in suchcases that the parallel processing in which a plurality of hardwarecomponents simultaneously operate can be considerably reduced by theaforementioned method.

Further, for example, the reception period (or transmission period) inthe first cell 10 may be determined so that it is included within thereception period (or the transmission period) in the second cell 20.Alternatively, the reception period (or the transmission period) in thesecond cell 20 may be determined so that it is included within thereception period (or the transmission period) in the first cell 10. Forexample, the reception gaps (or transmission gaps) in the first andsecond cells 10 and 20 may be configured so that the start point of thereception period (or the transmission period) in the second cell 20coincides with the start point of the reception period (or thetransmission period) in the first cell 10. According to this method, itis possible to reduce the delay in the data transmission/reception bydefining opportunities (or periods) in which the radio terminal 3 canreceive downlink data from the radio stations 1 and 2 (or transmituplink data to different radio stations) in an overlapped manner, andalso possible to prevent the radio stations 1 and 2 from simultaneouslyperforming data transmission/reception with the same radio terminal 3 asmuch as possible by having the radio stations 1 and 2 mutually haveinformation about the other station's data transmission/reception statewith the radio terminal 3. That is, this method provides flexibility tothe communication between the radio stations 1 and 2 and the radioterminal 3, thereby enabling them to determine whether a priority isgiven to the reduction in the delay in the data transmission/receptionor the reduction in the load on the radio terminal according to thecommunication policy.

The constraint information may be transmitted from the first radiostation 1 to the radio terminal 3 in the first cell 10, or transmittedfrom the second radio station 2 to the radio terminal 3 in the secondcell 20. For example, the first radio station 1 may transmit theconstraint information to the radio terminal 3 and the second radiostation 2. Alternatively, the first radio station 1 may transmit theconstraint information to the radio terminal 3 through the second radiostation 2. In this case, although the first radio station 1 transmits amessage containing the constraint information to the second radiostation 2, the second radio station 2 does not necessarily have torecognize the contents of that message. Alternatively, the second radiostation 2 may recognize the contents of the message. In the case wherethe second radio station 2 transmits the constraint information to theradio terminal 3 in the second cell 20, the second radio station maytransmit the constraint information as control information or maytransmit it in a manner similar to that for transmitting other downlinkdata. In another example, the first radio station 1 may transmit theconstraint information to the radio terminal 3 and the radio terminal 3may forward the constraint information to the second radio station 2. Instill another example, the second radio station 2 may transmitconstraint information related to the second cell 20 to the first radiostation 1 and the first radio station 1 may transmit the constraintinformation to the radio terminal 3. The transmission of constraintinformation between the first and second radio stations 1 and 2 may beperformed through the core network 4.

Next, configuration examples of the radio stations 1 and 2 and the radioterminal 3 according to this embodiment are explained. FIG. 2 is a blockdiagram showing a configuration example of the first radio station 1. Aradio communication unit 11 receives an uplink signal transmitted fromthe radio terminal 3 thorough an antenna. A reception data processingunit 13 restores the received uplink signal. The obtained reception datais forwarded to another network node such as a data transfer device or amobility management device in the core network 4, or to other radiostations through a communication unit 14. For example, uplink user datareceived from the radio terminal 3 is forwarded to a data transferdevice in a higher-layer network. Further, non-access stratum (NAS)control data among control data received from the radio terminal 3 isforwarded to a mobility management device in a higher-layer network.Further, the reception data processing unit 13 receives, from acommunication control unit 15, control data to be transmitted to theradio station 2, and transmits this control data to the radio station 2through the communication unit 14.

A transmission data processing unit 12 acquires user data destined forthe radio terminal 3 from the communication unit 14 and generates atransport channel by performing error correction encoding, ratematching, interleaving, and the like. Further, the transmission dataprocessing unit 12 generates a transmission symbol sequence by addingcontrol information to the data sequence of the transport channel. Theradio communication unit 11 generates a downlink signal by performingcarrier modulation based on the transmission symbol sequence, frequencyconversion, signal amplification, and the like, and transmits thegenerated downlink signal to the radio terminal 3. Further, thetransmission data processing unit 12 receives control data to betransmitted to the radio terminal 3 from the communication control unit15 and transmits this control data to the radio terminal 3 through theradio communication unit 11.

The communication control unit 15 controls inter-radio station carrieraggregation using the first and second cells 10 and 20. Further, in anexample, the communication control unit 15 may transmit theabove-described constraint information to the radio terminal 3.

FIG. 3 is a block diagram showing a configuration example of the secondradio station 2. The functions and the operations of a radiocommunication unit 21, a transmission data processing unit 22, areception data processing unit 23, and a communication unit 24 shown inFIG. 3 are similar to those of their corresponding elements shown inFIG. 2, i.e., those of the radio communication unit 11, the transmissiondata processing unit 12, the reception data processing unit 13, and thecommunication unit 14.

A communication control unit 25 controls inter-radio station carrieraggregation using the first and second cells 10 and 20. Further, in anexample, the communication control unit 25 may transmit theabove-described constraint information to the radio terminal 3.

FIG. 4 is a block diagram showing a configuration example of the radioterminal 3. A radio communication unit 31 supports carrier aggregationof a plurality of cells served by different radio stations, and is ableto simultaneously use the plurality of cells (e.g., the cells 10 and 20)for transmitting or receiving signals. Specifically, the radiocommunication unit 31 receives a downlink signal from one or both of theradio stations 1 and 2 through an antenna. A reception data processingunit 32 restores reception data from the received downlink signal andsends the restored reception data to a data control unit 33. The datacontrol unit 33 uses the reception data according to its purpose.Further, a transmission data processing unit 34 and a radiocommunication unit 31 generate an uplink signal by using transmissiondata supplied from the data control unit 33 and transmit the generateduplink signal to one or both of the radio stations 1 and 2.

A communication control unit 35, the radio terminal 3, controlsinter-radio station carrier aggregation which uses the first and secondcells 10 and 20. Further, the communication control unit 35 receives theconstraint information from the first radio station 1 or the secondradio station 2. Then, the communication control unit 35 performs areceiving operation (or transmitting operation) during the inter-radiostation carrier aggregation, in accordance with a reception constraint(or a transmission constraint) specified based on the constraintinformation.

Next, Procedure Examples 1 to 3 of a communication control method in aradio communication system according to this embodiment are explained.

Procedure Example 1

In Procedure Example 1, the first radio station 1 transmits theabove-described constraint information to the radio terminal 3 and thesecond radio station 2. FIG. 5 shows an example of a sequence diagramshowing a communication control method according to the ProcedureExample 1. In steps S101 and S102, the first radio station 1 transmitsconstraint information to the radio terminal 3 and the second radiostation 2. In a step S103, the first radio station 1 and the radioterminal 3 perform transmission/reception of a signal in the first cell10 according to a reception constraint (e.g., a reception gap) or atransmission constraint (e.g., a transmission gap) specified based onthe constraint information. In a step S104, the second radio station 2and the radio terminal 3 perform transmission/reception of a signal inthe second cell 20 according to a reception constraint (e.g., areception gap) or a transmission constraint (e.g., a transmission gap)specified based on the constraint information.

The constraint information transmitted to the radio terminal 3 mayinclude, for example, information about a reception period in which theradio terminal 3 should receive a predetermined downlink signal in thefirst cell 10 and information about a reception period in which theradio terminal 3 should receive a predetermined downlink signal in thesecond cell 20. The radio terminal 3 may receive predetermined downlinksignals on the first and second cells 10 and 20 only at the respectivereception periods specified based on the constraint information.Accordingly, it is possible to prevent the radio terminal 3 fromperforming unnecessary receiving operations (or decoding operations) fora predetermined downlink signal and thereby to prevent (or reduce)wasteful power consumption.

The constraint information may include only the information necessary tospecify the reception constraint (or transmission constraint) for eitherone of the first and second cells 10 and 20. For example, the firstradio station 1 may transmit, to the radio terminal 3 and the secondradio station 2, constraint information that is related only to thefirst cell 10 or related only to the second cell 20.

Further, the first radio station 1 may perform transmission or receptionregarding the radio terminal 3 in the first cell 10 while taking intoaccount the constraint information related to the second cell 20. Incontrast to this, the second radio station 2 may perform transmission orreception regarding the radio terminal 3 in the second cell 20 whiletaking into account the constraint information related to the first cell10. For example, the first radio station 1 may transmit a downlinksignal to the radio terminal 3 in the first cell 10 during a receptiongap for the radio terminal 3 in the second cell 20.

The constraint information transmitted from the first radio station 1 tothe radio terminal 3 may be the same as that transmitted from the firstradio station 1 to the second radio station 2, or may be different fromthat transmitted from the first radio station 1 to the second radiostation 2. For example, the first radio station 1 may transmitconstraint information related to both the first and second cells 10 and20 to the radio terminal 3 and transmit constraint information relatedonly to the second cell 20 to the second radio station 2.

Procedure Example 2

In Procedure Example 2, the first radio station 1 transmits theabove-described constraint information to the radio terminal 3 and theradio terminal 3 forwards that constraint information to the secondradio station 2. FIG. 6 shows an example of a sequence diagram showing acommunication control method according to the Procedure Example 2. In astep S201, the first radio station 1 transmits constraint information tothe radio terminal 3. In a step S202, the radio terminal 3 forwards theconstraint information to the second radio station 2. Processes in stepsS203 and S204 are similar to those in the steps S103 and S104 shown inFIG. 5.

The constraint information transmitted from the first radio station 1 tothe radio terminal 3 may include constraint information related to thefirst and second cells 10 and 20. In contrast to this, the constraintinformation transmitted from the radio terminal 3 to the second radiostation 2 may include only constraint information related to the secondcell 20. However, the constraint information transmitted from the radioterminal 3 to the second radio station 2 may be the same as thattransmitted from the first radio station 1 to the radio terminal 3. Theradio terminal 3 may voluntarily transmit the constraint informationrelated to the second cell 20 to the second radio station 2, or maytransmit the constraint information to the second radio station 2 inresponse to a report instruction sent from the second radio station 2.

Procedure Example 3

In Procedure Example 3, the constraint information is transmitted to theradio terminal 3 based on a request from the second radio station 2. Theconstraint information is generated by the first radio station 1 or thesecond radio station 2. FIG. 7 shows an example of a sequence diagramshowing a communication control method according to the ProcedureExample 3. In a step S301, the second radio station 2 sends, to thefirst radio station 1, a request for setting a constraint on signalreception or transmission performed by the radio terminal 3 in thesecond cell 20. In this process, the second radio station 2 may alsotransmit constraint information related to the second cell 20 to thefirst radio station 1.

In a step S302, the first radio station 1 sends, to the second radiostation 2, a response indicating whether or not the first radio station1 approves the constraint information related to the second cell 20,that is, whether or not the first radio station 1 approves theapplication of the constraint for the second cell. In the case where thefirst radio station 1 does not receive constraint information from thesecond radio station 2 when the first radio station 1 receives therequest in the step S301, the first radio station 1 may transmitconstraint information related to the second cell 20 to the second radiostation 2. When the first radio station 1 approves the application ofthe constraint for the second cell 20, the first radio station 1transmits to the radio terminal 3 the constraint information related tothe second cell 20 (step S303). Processes in steps S304 and S305 aresimilar to those in the steps S103 and S104 shown in FIG. 5.

In a step S302, the first radio station 1 may also transmit, to thesecond radio station 2, constraint information related to the first cell10 for the radio terminal 3. Similarly, in a step S303, the first radiostation 1 may also transmit, to the radio terminal 3, constraintinformation related to the first cell 10.

When the first radio station 1 has received constraint informationrelated to the second cell from the second radio station 2, the firstradio station 1 may, instead of accepting the constraint informationreceived from the second radio station 2, generate (or configure) newconstraint information related to the second cell and transmit thegenerated (or configured) constraint information to the second radiostation 2.

Modification of Procedure Example 3

The first radio station 1 may request the second radio station 2 togenerate (or configure) constraint information related to the secondcell 20. In this case, the second radio station 2 may transmitconstraint information related to the second cell 20 to the first radiostation 1 and the first radio station 1 may transmit the receivedconstraint information to the radio terminal 3.

Second Embodiment

In this embodiment, an example where the above-described firstembodiment is applied to a 3GPP LTE system is explained. A configurationexample of a radio communication system according to this embodiment maybe similar to that shown in FIG. 1. Note that the radio stations 1 and 2correspond to eNBs, the radio terminal 3 corresponds to an UE, and thecore network 4 corresponds to an EPC (Evolved Packet Core). Transmissionand reception of information between radio stations (i.e., between eNBs)may use an X2 interface, which is a direct interface, may use an S1interface through the EPC, or may use a newly-defined interface (e.g.,an X3 interface). A radio terminal (UE) 3 supports carrier aggregationof a plurality of cells served by different radio stations (eNBs)(called “Inter-eNB CA” or “Inter-Site CA”). Note that the “Inter-eNB CA”in this specification is not limited to actual simultaneous reception ortransmission of signals on the cells of different eNBs. That is, itincludes: a state where the radio terminal (UE) actually receives ortransmits a signal (e.g., user data or control information) in eitherone of the cells of different eNBs although the radio terminal is ableto receive or transmit in both of the cells of different eNB; a statewhere the radio terminal receives or transmits signals of differenttypes in the respective cells of different eNBs; and a state where theradio terminal uses each of the cells of different eNBs for eithersignal reception or signal transmission.

The following explanation is given on the assumption that: the radiostations 1 and 2 are eNBs 1 and 2; the radio terminal 3 is an UE 3; andthe core network 4 is an EPC 4. Further, it is assumed that the UE 3performs inter-radio base station carrier aggregation (Inter-eNB CA) inwhich the UE 3 uses the cell 20 of the eNB 2 as a secondary cell (SCell)while the UE 3 is already using the cell 10 of the eNB 1 as a primarycell (PCell). Note that the primary cell (PCell) is a cell that hasalready been used since before the CA is started. In contrast to this,the second cell (SCell) is a cell that is used (activated) in additionto the PCell or in dependence thereon on the precondition that the UE 3is already connected to the primary cell. Non-Access Stratum (NAS)mobility information, security information (or security input), and thelike are transmitted and received through the PCell when a radioconnection is established (i.e., at the time of RRC ConnectionEstablishment) or reestablished (i.e., at the time of RRC ConnectionRe-establishment). A DL Component Carrier used for the PCell is a DLPCC, and an UL Component Carrier used for the PCell is an UL PCC.Similarly, a DL Component Carrier used for the SCell is a DL SCC, and anUL Component Carrier used for the SCell is an UL SCC.

The eNB 1 or 2 transmits constraint information to the UE 3. Theconstraint information contains information necessary to specify atleast one of a reception constraint and transmission constraint for atleast one of the PCell and the SCell when the SCell (second cell 20) isactivated for the inter-eNB carrier aggregation. The UE 3 receives theconstraint information from the eNB 1 or 2. Then, the UE 3 performs areceiving operation in accordance with a reception constraint or atransmitting operation in accordance with a transmission constraint, inthe inter-eNB carrier aggregation.

For example, in order to specify at least one of the receptionconstraint and the transmission constraint, the constraint informationmay explicitly or implicitly indicate at least one of the followingitems (a) to (h):

(a) A period (or timing) in/at which a predetermined downlink signal (DLSignal) should be received;

(b) A period (or timing) in/at which a predetermined downlink signalshould be decoded;

(c) A period (or timing) in/at which a predetermined downlink signaldoes not have to be received;

(d) A period (or timing) in/at which a predetermined downlink signaldoes not have to be decoded;

(e) A period (or timing) in/at which the transmission of an uplinksignal (UL Signal) is prohibited;

(f) A period (or timing) in/at which an uplink signal can betransmitted;

(g) A period (or timing) in/at which an uplink signal may betransmitted; and

(h) A period (or timing) in/at which the transmission of an uplinksignal may be permitted.

Although the above-described periods or timings are expressed from thestandpoint of the UE 3, they have the same meanings even when they areexpressed from the standpoint of the eNBs 1 and 2.

The predetermined downlink signal (DL signal) is, for example, a signal(L1/2 control signaling) transmitted through a PDCCH or an EnhancedPDCCH (EPDCCH), which is a downlink control channel. This signalincludes scheduling information regarding downlink radio resourcesallocated for data transmission to the UE or the like, or use permissioninformation (UL grant) of uplink radio resources used for datatransmission or the like by the UE. Alternatively, the predetermineddownlink signal (DL signal) may be a signal for transmitting Paging or asignal for transmitting System information.

Further, examples of possible reception constraints also include asetting of “reception gap” indicating the above-described item (c) or(d). Alternatively, it is also conceivable to indicate “DiscontinuousReception (DRX)” that is specified by a combination of theabove-described items (a) and (c) or items (b) and (d). Note that whenDRX is indicated, an entirely-same setting or a partially-same settingmay be used for both the first cell 10 (e.g., the PCell) and the secondcell 20 (e.g., the SCell) as the setting related to the DRX.Alternatively, the whole setting related to the DRX may be independentlyset to the first cell 10 and the second cell 20. Examples of conceivablesettings related to the DRX include On-Duration (OnDurationTimer),drx-InactivityTimer, drx-RetransmissionTimer, longDRX-CycleStartOffset,shortDRX-Cycle, and drxShortCycleTimer, HAM) RTT Tinier. Further, theUE3 may operate all or some of the above-described timers used for theDRX in a cooperative manner for the first and second cells 10 and 20.Alternatively, the UE3 may operate them independently for the first andsecond cells 10 and 20. In this way, the DRX can be performed in aflexible manner according to the data transmission state in each of thefirst and second cells 10 and 20. Although, in general, the operation oftimers used for the DRX is performed in the radio terminal, acorresponding operation may be performed in the radio base station.

As an example of the transmission constraint, it is conceivable to set“transmission gap” indicating the above-described item (e).Alternatively, it is also conceivable to indicate “masking oftransmission timing” that is specified by a combination of the item andone of the items (f) to (h), i.e., narrows down the timing (e.g.,subframe) at which the transmission of an uplink signal is permitted.

Further, the constraint information may directly or implicitly indicateat least one of the reception constraint and the transmissionconstraint. For example, the constraint information may include at leastone of the following items:

(A) A request (or instruction) of application of a reception constraint;

(B) A request (or instruction) of application of a transmissionconstraint;

(C) Contents of a reception constraint;

(D) Contents of a transmission constraint;

(E) Control parameters regarding a reception constraint; and

(F) Control parameters regarding a transmission constraint.

The item (A) is used, for example, when the application of a receptionconstraint is requested in a state where the eNBs 1 and 2, or the eNBs 1and 2 and the UE 3 have common information indicating what kind ofconstraint is indicated by the reception constraint, or that informationhas been defined in the specifications. Similarly, the item (B) is used,for example, when the application of a transmission constraint isrequested in a state where the eNBs 1 and 2 have common informationindicating what kind of constraint is indicated by the transmissionconstraint, or that information has been defined in the specifications.

The item (C) is information indicating what kind of reception constraintis applied. For example, the item (C) indicates the setting of“reception gap”, the instruction of “DRX”, or the like. Similarly, theitem (D) is information indicating what kind of transmission constraintis applied. For example, the item (D) indicates the setting of“transmission gap”, the instruction for “masking of transmissiontiming”, or the like.

The item (E) is information indicating a control parameter(s) thatshould be set in the reception constraint. For example, the item (E)indicates at least one of the above-described items (a) to (d).Similarly, the item (F) is information indicating a control parameter(s)that should be set in the transmission constraint. For example, the item(F) indicates at least one of the above-described items (e) to (h).

The constraint information may be transmitted from the eNB 1 to the UE 3in the first cell 10, or transmitted from the eNB 2 to the UE 3 in thesecond cell 20. For example, the eNB 1 may transmit the constraintinformation to the UE 3 and the eNB 2. Further, the eNB 1 may transmitthe constraint information to the UE 3 through the eNB 2. In this case,although the eNB 1 transmits a message containing the constraintinformation to the eNB 2 through an X2 interface (or a new interface),the eNB 2 does not necessarily have to recognize the contents of themessage. Alternatively, the eNB 2 may recognize the contents of themessage. In the case where the eNB 2 transmits the constraintinformation to the UE 3 in the second cell 20, the eNB 2 may transmitthe constraint information by using a Signaling Radio Bearer (SRB) ascontrol information or may transmit it by using a Data Radio Bearer(DRB) in a manner similar to that for transmitting other downlink data.In other examples, the eNB 1 may transmit the constraint information tothe UE 3 and the UE 3 may forward the constraint information to the eNB2. Further, in other examples, the eNB 2 may transmit constraintinformation related to the second cell 20 to the eNB 1 and the eNB 1 maytransmit the constraint information to the UE 3. The transmission ofconstraint information between the eNB 1 and the eNB2 may be performedthrough the core network 4 by using an S1 interface.

The constraint information may be applied to both of the PCell (firstcell 10) and the SCell (second cell 20), or may be applied to either oneof them.

The reception constraint (or the transmission constraint) for the SCellbased on the constraint information may be configured independently ofthat for the PCell. Alternatively, the reception constraint (or thetransmission constraint) of the SCell may be configured so as to bedependent on that for the PCell. For example, the reception gap (or thetransmission gap) in the SCell may be configured so that at least a partof the reception gap (or the transmission gap) in the SCell does notoverlap that of the PCell. For example, the reception gaps in the PCelland the SCell may be configured so that the start point of the receptionperiod in the SCell does not coincide with the start point of thereception period in the PCell. Similarly, the transmission gaps in thePCell and the SCell may be configured so that the start point of thetransmission period in the SCell does not coincide with the start pointof the transmission period in the PCell.

Further, for example, the reception gaps (or the transmission gaps) inthe PCell and the SCell may be determined so that the reception period(or the transmission period) in the PCell does not overlap that in theSCell at all.

Further, for example, the reception gap (or the transmission gap) in theSCell may be determined so that it is included within the reception gap(or the transmission gap) in the PCell. In other words, the receptionperiod (or the transmission period) in the PCell may be determined sothat it is included within the reception period (or the transmissionperiod) in the SCell. Alternatively, the reception gap (or thetransmission gap) in the PCell may be determined so that it is includedwithin the reception gap (or the transmission gap) in the SCell. Inother words, the reception period (or the transmission period) in theSCell may be determined so that it is included within the receptionperiod (or the transmission period) in the PCell.

Procedure Examples 4 to 8 of a communication control method performed ina radio communication system according to this embodiment are explainedhereinafter. In Procedure Examples 4 to 6, example cases where areception gap (Rx gap) specified based on constraint informationindicates a period (one or more subframes) in which a PDCCH does nothave to be decoded are explained. Further, in the following explanation,a period in which a PDCCH should be received and decoded is called“ON-period (or Active period) and a period in which a PDCCH does nothave to be decoded is called “OFF-period (or Inactive period). Forexample, the OFF-period corresponds to a reception gap (i.e., a periodin which a reception gap is effective).

Procedure Example 4

Procedure Example 4 corresponds to the Procedure Example 1 explained inthe first embodiment. That is, the eNB 1 transmits, to the UE 3 and theeNB 2, constraint information necessary to specify a reception gap. FIG.8 shows an example of a sequence diagram showing the Procedure Example4. Note that in FIG. 8, the first and second cells 10 and 20 areexpressed as “CELL1” and “CELL2”, respectively. In a step S401, the eNB1 transmits a message (RRC Connection Reconfiguration) containingconstraint information to the UE 3. In a step S402, the eNB 1 transmitsa message (Connection indication) containing the constraint informationto the eNB 2.

In steps S403 to S406, the eNBs 1 and 2 transmit downlink data (DL data)in the CELL1 and the CELL2, respectively, based on the constraintinformation, and the UE 3 receives the DL data transmitted from the eNBs1 and 2. For example, in a reception period (ON-period) in the CELL 1,the eNB 1 transmits DL data for the UE 3 (e.g., VoIP data) and the UE 3can receive the DL data (e.g., VoIP data) by decoding the PDCCH (stepsS403 and S405). Meanwhile, in a reception period (ON-period) in theCELL2, the eNB 2 transmits DL data for the UE 3 (e.g., FTP data) and theUE 3 can receive the DL data (e.g., FTP data) by decoding the PDCCH(steps S404 and S406).

The UE 3 does not need to decode the PDCCH of the CELL 1 in a receptiongap (OFF-period) for the CELL1. Similarly, the UE 3 does not need todecode the PDCCH of the CELL 2 in a reception gap (OFF-period) for theCELL2. In this way, it is possible to avoid unnecessary PDCCH reception(or decoding) and thereby to reduce the power consumption of the UE 3.In FIG. 8, the reception gap (OFF-period) in the CELL1 corresponds tothe whole period in the CELL1 except for the ON-period. Similarly, thereception gap (OFF-period) in the CELL2 corresponds to the whole periodin the CELL2 except for the ON-period.

The constraint information may include only the information necessary tospecify the reception gap (OFF-period) in either one of the CELL1 andthe CELL2. For example, the eNB 1 may transmit, to the UE 3 and the eNB2, constraint information that is related only to the CELL1 or relatedonly to the CELL2.

Further, the eNB 1 may perform transmission or reception regarding theUE 3 in the CELL1 while taking into account the constraint informationrelated to the CELL2. In contrast to this, the eNB 2 may performtransmission or reception regarding the UE 3 in the CELL2 while takinginto account the constraint information related to the CELL1. Forexample, the eNB 1 may transmit a downlink signal to the UE 3 in theCELL1 during a reception gap (OFF-period) for the UE 3 in the CELL2.

The constraint information transmitted from the eNB 1 to the UE 3 may bethe same as that transmitted from the eNB 1 to the eNB 2, or may bedifferent from that transmitted from the eNB 1 to the eNB 2. Forexample, the eNB 1 may transmit constraint information related to boththe CELL1 and the CELL2 to the UE 3 and transmit constraint informationrelated only to the CELL2 to the eNB 2.

The constraint information (i.e., a reception constraint or/and atransmission constraint specified by the constraint information) may bemade effective at the moment when the UE or the eNB2 receives thatconstraint information from the eNB1 or when a predetermined time haselapsed after the UE or the eNB2 receives that constraint information.The predetermined time may be specified by transmitting it with theconstraint information, or may be configured in advance in the UE 3 andthe eNB 2.

The constraint information may be made effective, for example, inaccordance with the below-shown procedure. FIG. 9 shows Options 1 to 3of a procedure for making the constraint information effective.Processes in steps S401 and S402 in FIG. 9 are similar to those in thesteps S401 and S402 shown in FIG. 8. Steps S411 and S412 in FIG. 9represent the Option 1. That is, in the step S411, the eNB 1 transmitsto the UE 3 an instruction for making constraint information related tothe CELL1 effective (Activation of constraint on CELL1). In the stepS412, the eNB 2 transmits to the UE 3 an instruction for makingconstraint information related to the CELL2 effective (Activation ofconstraint on CELL2).

Steps S421 and S422 in FIG. 9 represent the Option 2. In the step S421,the eNB 1 transmits to the UE 3 an instruction for making constraintinformation related to the CELL1 and the CELL 2 effective (Activation ofconstraint on CELL1 and CELL2). At this point, as shown in the stepS422, the eNB 1 may notify the eNB 2 that the eNB 1 has made theconstraint information effective for the UE 3 (Constraint activationindication).

Steps S431 and S432 in FIG. 9 represent the Option 3. In the step S431,the eNB 1 transmits to the UE 3 an instruction for making constraintinformation related to the CELL1 and the CELL 2 effective (Activation ofconstraint on CELL1 and CELL2). In the step S432, the UE 3 notifies theeNB 2 that the constraint information for the CELL2 has become effective(Constraint activation report).

Procedure Example 5

Procedure Example 5 corresponds to the Procedure Example 2 explained inthe first embodiment. That is, the eNB 1 transmits constraintinformation to the UE 3 and the UE 3 forwards the constraint informationto the eNB 2. FIG. 10 shows an example of a sequence diagram showing theProcedure Example 5. In FIG. 10, the first and second cells 10 and 20are expressed as “CELL1” and “CELL2”, respectively. In a step S501, theeNB 1 transmits a message (RRC Connection Reconfiguration) containingconstraint information to the UE 3. In a step S502, the UE 3 transmits amessage (Constraint information report) containing constraintinformation to the eNB 2. The UE 3 may voluntarily transmit theconstraint information related to the CELL2 to the eNB 2, or maytransmit the constraint information to the eNB 2 in response to aconstraint information report instruction (Constraint information reportrequest) sent from the eNB 2. Processes in steps S503 and S505 in FIG.10 are similar to those in the steps S403 and S404 shown in FIG. 8.

The constraint information transmitted from the eNB 1 to the UE 3 mayinclude constraint information related to the CELL1 and the CELL2. Incontrast to this, the constraint information transmitted from UE 3 tothe eNB 2 may include only constraint information related to the CELL2.However, the constraint information transmitted from UE 3 to the eNB 2may be the same as that transmitted from the eNB 1 to the UE 3.

Procedure Example 6

Procedure Example 6 corresponds to the Procedure Example 3 explained inthe first embodiment. That is, the constraint information is transmittedto the UE 3 based on a request from the eNB 2. The constraintinformation is generated by the eNB 1 or the eNB 2. FIG. 11 shows anexample of a sequence diagram showing a communication control methodaccording to the Procedure Example 6. In FIG. 11, the first and secondcells 10 and 20 are expressed as “CELL1” and “CELL2”, respectively. In astep S601, the eNB 2 sends to the eNB 1 a request for setting aconstraint (i.e., a reception constraint or a transmission constraint)on signal reception or transmission performed by the UE 3 in the CELL2(Constraint configuration request (for CELL2)).

In a step S602, the eNB 1 sends to the eNB 2 a response indicatingwhether or not the eNB 1 approves the constraint information related tothe CELL2, that is, whether or not the eNB approves the application ofthe constraint for the CELL2. When the eNB 1 approves the application ofthe constraint for the CELL2, the eNB 1 transmits to the eNB 2 theconstraint information related to the CELL2 (Constraint informationindication). Further, in a step S603, the eNB 1 transmits to the UE 3the constraint information related to the CELL2 (RRC ConnectionReconfiguration (including Constraint information)). In a step S603, theeNB 1 may also transmit to the UE 3 the constraint information relatedto the CELL 1. Processes in steps S604 and S605 are similar to those inthe steps S403 and S404 shown in FIG. 8.

When the eNB 2 transmits the request in the step S601, the eNB 2 mayalso transmit constraint information that the eNB 2 recommends. In thiscase, the eNB 1 may send to the eNB 2 a result of the determination ofwhether the constraint information received from the eNB 2 has beenapproved or not. Alternatively, the eNB 1 may generate (or configure)constraint information that the eNB 1 recommends instead of theconstraint information received from the eNB 2, and transmit thegenerated (or configured) constraint information to the eNB 2.

In a step S602 the eNB 1 may also transmit, to the eNB 2, constraintinformation related to the CELL1 for the UE 3. Similarly, in a stepS603, the eNB 1 may transmit, to the UE 3, constraint informationrelated to the CELL1.

Modification of Procedure Example 6

The eNB 1 may request the eNB 2 to generate (or configure) constraintinformation related to the CELL2. In this case, the eNB 2 may transmitconstraint information related to the CELL2 to the eNB and the eNB 1 maytransmit the received constraint information to the UE 3. FIG. 12 showsan example of a sequence diagram showing a modification of the ProcedureExample 6. In FIG. 12, the first and second cells 10 and 20 areexpressed as “CELL1” and “CELL2”, respectively. In a step S611, the eNB1 requests the eNB 2 to generate (or configure) constraint informationrelated to the CELL2 (Constraint configuration request (for CELL2)). Ina step S612, the eNB 2 transmits to the eNB 1 the constraint informationrelated to the CELL2 (Constraint information indication). In a stepS613, the eNB 1 may transmit to the eNB 2 a response indicting that theeNB 1 has received the constraint information related to the CELL2. In astep S614, the eNB 1 transmits to the UE 3 the constraint informationrelated to the CELL2, which has been received from the eNB 2 (RRCConnection Reconfiguration (including Constraint information)).

Procedure Example 7

In this example, Options 1 to 3 of the setting of reception periods(ON-periods) and non-reception periods (OFF-periods) in the CELL1 andthe CELL2 are explained. In the Option 1, the non-reception periods(OFF-periods) in the PCell and the SCell for the UE 3 are configured sothat the reception periods (ON-periods) in the CELL1 do not overlap withthe reception periods (ON-periods) in the CELL2 at all. Note that thenon-reception periods (OFF-periods) in the PCell may partially overlapwith the non-reception periods (OFF-periods) in the SCell. In otherwords, a non-reception period (OFF-period) in which the PDCCH does notneed to be received in any of the CELL1 and the CELL2 may be configured.FIG. 13A is a sequence diagram showing a specific example of downlinkdata transmission in the Option 1. FIG. 13B shows an example of anarrangement of reception periods (ON-periods) and non-reception periods(OFF-periods) in the Option 1. As shown in FIGS. 13A and 13B, thereception periods (ON-periods) in the CELL1 and those in the CELL2 arealternately arranged along the time axis. According to the Option 1, theUE 3 does not need to simultaneously receive signals in the CELL1 andthe CELL2, i.e., receives a signal in only one of them.

In the Option 2, the non-reception periods (OFF-periods) in the CELL1for the UE 3 and those in the CELL2 are configured independently of eachother. In other words, the reception periods (ON-periods) in the CELL1for the UE 3 and those in the CELL2 are configured independently of eachother. Note that in the Option 2, a non-reception period(s)(OFF-period(s)) may be configured for only one of the CELL1 and theCELL2. FIG. 14A is a sequence diagram showing a specific example ofdownlink data transmission in the Option 2. FIG. 14B shows an example ofan arrangement of reception periods (ON-periods) and non-receptionperiods (OFF-periods) in the Option 2. Since reception periods(ON-periods) and non-reception periods (OFF-periods) in the CELL1 areindependently configured from those in the CELL2, the reception periods(ON-periods) in the CELL1 and those in the CELL2 may partially overlapeach other as shown in FIGS. 14A and 14B. Further, by chance, thereception periods (ON-periods) in the CELL1 and those in the CELL2 maynot overlap at all. According to the Option 2, the reception periods(ON-periods) and the non-reception periods (OFF-periods) for the CELL 1and the CELL2 can be optimized according to their respectivetransmission patterns.

In the Option 3, the reception periods (ON-periods) in the PCell areconfigured so that they are included within the reception periods(ON-periods) in the SCell. FIG. 15A is a sequence diagram showing aspecific example of downlink data transmission in the Option 3. FIG. 15Bshows an example of an arrangement of reception periods (ON-periods) andnon-reception periods (OFF-periods) in the Option 3. Note that incontrast to the example shown in FIGS. 15A and 15B, the receptionperiods (ON-periods) in the SCell may be determined so that they areincluded within the reception periods (ON-periods) in the PCell.

Note that in this embodiment, for example in the above-described Options1 to 3, it is conceivable that each non-reception period (OFF-period)corresponds to a reception gap (Rx gap) in a serving cell.Alternatively, it is conceivable that each reception period (ON-period)and each non-reception period (OFF-period) correspond to an On-Duration(Wake up period) and an Opportunity for DRX (Sleep period),respectively, of a DRX cycle in a serving cell.

Procedure Example 8

In the above-described Procedure Examples 4 to 7, the setting ofreception periods (ON-periods) and non-reception periods (OFF-periods)for the downlink transmission in inter-eNB carrier aggregation isexplained. In the Procedure Example 8, an example where a transmissionperiod and a non-transmission period are configured based on constraintinformation is explained. As described previously, the transmissionperiod is an ON-period (or an Active period) in which the UE 3 may bepermitted a transmission of an uplink signal (UL signal). In contrast tothis, the non-transmission period is an OFF-period (or an Inactiveperiod) in which the UE 3 is prohibited from transmitting an uplinksignal (UL signal). Note that it is conceivable that thenon-transmission period (OFF-period) in a serving cell corresponds to atransmission gap (Tx gap). Alternatively, it is conceivable that thetransmission period (ON-period) in a serving cell corresponds to atransmission available period in transmission timing masking. Further,it is conceivable that the transmission period (ON-period) and thenon-transmission period (OFF-period) in a serving cell correspond to anOn-Duration (Transmission period) and an Opportunity for DTX(Non-transmission period), respectively, of a DTX cycle.

FIG. 16 shows an example of a sequence diagram showing the ProcedureExample 8. In FIG. 16, the first and second cells 10 and 20 areexpressed as “CELL1” and “CELL2”, respectively. In a step S801, the eNBs1 and 2 and the UE 3 configure transmission periods (ON-periods) andnon-transmission periods (OFF-periods) in the CELL1 and the CELL2 basedon constraint information. The transmission periods (ON-periods) and thenon-transmission periods (OFF-periods) may be configured in only one ofthe CELL1 and the CELL2. In the step S801, the UE 3 receives theconstraint information from the eNB 1 or the eNB 2. The process in thestep S801 may be similar to one of the constraint informationtransmission/reception procedures explained in the Procedure Examples 4to 6.

In steps S802 to S405, the UE 3 transmits uplink signals in the CELL1and the CELL2 based on the constraint information and the eNBs 1 and 2receive the uplink signals transmitted from the UE. In the example shownin FIG. 16, RRC signaling is transmitted on the CELL1 and uplink data(user data) is transmitted on the CELL2. For example, the UE 3 transmitsRRC signaling to the eNB 1 during a transmission period (ON-period) inthe CELL1 (steps S802 and S804). Further, the UE 3 transmits uplink datato the eNB 2 during a transmission period (ON-period) in the CELL2(steps S803 and S805).

In FIG. 16, transmission periods and non-transmission periods(OFF-periods) in the CELL1 and the CELL2 are configured in accordancewith a concept similar to that of the Option 1 explained above in theProcedure Example 7. That is, the non-transmission periods (OFF-periods)in the PCell and the SCell for the UE 3 are configured so that thetransmission periods (ON-periods) in the CELL1 do not overlap with thetransmission periods (ON-periods) in the CELL2 at all. Note that thenon-transmission periods (OFF-periods) in the PCell may partiallyoverlap with the non-transmission periods (OFF-periods) in the SCell. Inother words, a non-transmission period (OFF-period) in which uplinktransmission is prohibited in both of the CELL1 and the CELL2 may beconfigured. In this way, the UE 3 does not need to simultaneouslytransmit uplink signals (RRC signaling and Data) in both the CELL1 andthe CELL2, and it is thus possible to alleviate the complexity. Inparticular, when the CELL1 and the CELL2 use different frequency bands,it is conceivable that the uplink signal transmission timings suitablefor maintaining the uplink synchronization are different from eachother, thus requiring control in which this transmission timingdifference is taken into account. However, by configuring transmissionperiods (ON-periods) as in the example shown in the steps S802 to S805in FIG. 16, the complexity of this control is alleviated.

The above-described Procedure Examples 4 to 8 may be modified, forexample, as shown below. In the Procedure Examples 4 to 8, it is shownthat a reception gap (Rx gap) may be configured as an example of anon-reception period based on the constraint information, and atransmission gap (Tx gap) may be configured as an example of anon-transmission period based on the constraint information. In suchcases, the below-listed exception handling processes may be applied.

-   The UE 3 receives important information such as system information    (Master Information Block (MIB) or System Information Block (SIB)),    paging (Paging indication or Paging channel (PCH)), and RRC    signaling, irrespective of the reception gap.-   The UE 3 receives a downlink re-transmitted signal, irrespective of    the reception gap.-   The UE 3 transmits an uplink re-transmitted signal, irrespective of    the transmission gap.

The constraint information may be transmitted to the UE 3 through RRCsignaling, and an instruction for activating or deactivating theconstraint information (i.e., a reception constraint or a transmissionconstraint) may be transmitted through Medium Access Control (MAC)signaling or Layer 1 and/or Layer 2 (L1/L2) control signaling.

The transmission of constraint information may be performed by any oneof the radio base stations (e.g., the eNB 1), and the activation and thedeactivation of that constraint information (i.e., a receptionconstraint or a transmission constraint) may be performed in each of theradio base stations (the eNBs 1 and 2).

The transmission/reception of constraint information between the eNB 1and the eNB 2 may be performed through a core network (e.g., an EPC).

Other Embodiments

In the first and second embodiments, examples where a receptionconstraint or a transmission constraint based on the constraintinformation is individually configured for each radio terminal (UE) areshown. However, a reception constraint or a transmission constraintbased on the constraint information may be used in common for aplurality of radio terminals (UEs), or may be used in common for all theradio terminals (UEs) located in the same cell. Further, the constraintinformation may be individually transmitted to each radio terminal (UE).Alternatively, the constraint information may be transmitted to aplurality of radio terminals (UEs) by using a common massage, or may betransmitted to all the radio terminals (UEs) located in the same cell byusing a common massage by using system information).

The first and second embodiments can be applied to a case where thefirst radio station 1 (eNB 1) is a macro radio base station (Macro eNB(MeNB)) that serves (manages) a macro cell having a relatively largecoverage and the second radio station 2 (eNB 2) is a low-power radiobase station (Low Power Node (LPN)) that serves (manages) a cell havinga small coverage. Examples of a LPN include a pico-radio base station(Pico eNB (PeNB)) having functions (protocol layers) similar to those ofthe MeNB and a new type of network node (New Node) having fewerfunctions (protocol layers) than those of the MeNB. Alternatively, it isconceivable to employ a configuration in which a MeNB manages a LPN andcontrol functions (e.g., an RRC layer) in an LPN cell. Further, thesecond cell 20 may be a new type of cell (New Cell Type) which isdifferent from conventional cells and uses a new type of carrier (NewCarrier Type) different from conventional carriers.

Each of the communication control methods performed by the radio station1 (communication control unit 15), the radio station 2. (communicationcontrol unit 25), and the radio terminal 3 (communication control unit35) described in the first and second embodiments may be implemented byusing a semiconductor processing device such as an Application SpecificIntegrated Circuit (ASIC). Alternatively, these methods may beimplemented by causing a computer system including at least oneprocessor (e.g., Microprocessor, Micro Processing Unit (MPU), or DigitalSignal Processor (DSP)) to execute a program. Specifically, one or moreprograms including instructions for causing a computer system to performalgorithms shown in the flowcharts and the sequence diagrams may becreated, and these programs may be supplied to a computer.

These programs can be stored in various types of non-transitory computerreadable media and thereby supplied to computers. The non-transitorycomputer readable media includes various types of tangible storagemedia. Examples of the non-transitory computer readable media include amagnetic recording medium (such as a flexible disk, a magnetic tape, anda hard disk drive), a magneto-optic recording medium (such as amagneto-optic disk), a CD-ROM (Read Only Memory), a CD-R, and a CD-R/W,and a semiconductor memory (such as a mask ROM, a PROM (ProgrammableROM), an EPROM (Erasable PROM), a flash ROM, and a RAM (Random AccessMemory)). Further, these programs can be supplied to computers by usingvarious types of transitory computer readable media. Examples of thetransitory computer readable media include an electrical signal, anoptical signal, and an electromagnetic wave. The transitory computerreadable media can be used to supply programs to computer through a wirecommunication path such as an electrical wire and an optical fiber, orwireless communication path.

In the first and second embodiments, an LTE system has been mainlyexplained. However, these embodiments may be applied to radiocommunication systems other than the LTE system, such as a 3GPPUniversal Mobile Telecommunications System (UMTS), a 3GPP2 CDMA2000system (1×RTT, High Rate Packet Data (HRPD)), a Global System for MobileCommunications (GSM) system, and a WiMAX system.

Further, the above-described embodiments are merely examples for theapplication of the technical ideas obtained by the present inventors.Needless to say, these technical ideas are not limited to theabove-described embodiments and various modifications can be madethereto.

This application is based upon and claims the benefit of priority fromJapanese patent applications No. 2013-033703, filed on Feb. 22, 2013,the disclosure of which is incorporated herein in its entirety byreference.

REFERENCE SIGNS LIST

-   1 FIRST RADIO STATION-   2 SECOND RADIO STATION-   3 RADIO TERMINAL-   10 FIRST CELL-   20 SECOND CELL-   15 COMMUNICATION CONTROL UNIT-   25 COMMUNICATION CONTROL UNIT-   35 COMMUNICATION CONTROL UNIT

The invention claimed is:
 1. A radio communication system comprising: afirst radio station configured to serve a first cell; a second radiostation configured to serve a second cell; and a radio terminalconfigured to support Dual Connectivity using the first and secondcells, wherein the first radio station is configured to send firstconstraint information to the second radio station and receive secondconstraint information from the second radio station, the first radiostation is further configured to transmit uplink constraint informationto the radio terminal, wherein the first constraint information is aconstraint related to at least one of uplink signal transmission by theradio terminal and downlink signal transmission by the first radiostation in the first cell, the second constraint information is aconstraint related to at least one of uplink signal transmission by theradio terminal and downlink signal transmission by the second radiostation in the second cell, the uplink constraint information containsan information element for deriving transmission constraint in a casewhere the Dual Connectivity using the first and second cells isperformed, the transmission constraint is a constraint related to uplinksignal transmission by the radio terminal over one or more subframeperiods and indicates a first timing in which uplink signal transmissionin the first cell is allowed, a second timing in which uplink signaltransmission in the second cell is allowed is set so that the firsttiming and the second timing are not overlapped, and the radio terminalis configured to perform uplink transmission in the first cell and thesecond cell based on the uplink constraint information.
 2. The radiocommunication system according to claim 1, wherein the uplink constraintinformation contains an information element for deriving start of thefirst timing.
 3. The radio communication system according to claim 1,wherein the second cell is a second type of cells which is differentfrom a first type of cells of the first cell.
 4. A radio terminalcomprising: a memory storing instructions; and at least one processorconfigured to process the instructions to: support Dual Connectivityusing a first cell served by a first radio station and a second cellserved by a second radio station; and receive uplink constraintinformation from the first radio station, wherein the uplink constraintinformation contains an information element for deriving transmissionconstraint in a case where the Dual Connectivity using the first andsecond cells is performed, the transmission constraint is a constraintrelated to uplink signal transmission by the radio terminal over one ormore subframe periods and indicates a first timing in which uplinksignal transmission in the first cell is allowed, a second timing inwhich uplink signal transmission in the second cell is allowed is set sothat the first timing and the second timing are not overlapped, and theradio terminal is configured to perform uplink transmission in the firstcell and the second cell based on the uplink constraint information. 5.The radio terminal according to claim 4, wherein the uplink constraintinformation contains an information element for deriving start of thefirst timing.
 6. The radio terminal according to claim 4, wherein thesecond cell is a second type of cells which is different from a firsttype of cells of the first cell.
 7. A first radio station configured toserve a first cell, comprising: a memory storing instructions; and atleast one processor configured to process the instructions to: supportDual Connectivity using the first cell and a second cell served by asecond radio station; send first constraint information to the secondradio station; receive second constraint information from the secondradio station; and transmit uplink constraint information to the radioterminal, wherein the first constraint information is a constraintrelated to at least one of uplink signal transmission by the radioterminal and downlink signal transmission by the first radio station inthe first cell, the second constraint information is a constraintrelated to at least one of uplink signal transmission by the radioterminal and downlink signal transmission by the second radio station inthe second cell, the uplink constraint information contains aninformation element for deriving transmission constraint in a case wherethe Dual Connectivity using the first and second cells is performed, thetransmission constraint is a constraint related to uplink signaltransmission by the radio terminal over one or more subframe periods andindicates a first timing in which uplink signal transmission in thefirst cell is allowed, and the transmission constraint causes the radioterminal to set a second timing in which uplink signal transmission inthe second cell is allowed so that the first timing and the secondtiming are not overlapped.
 8. The first radio station according to claim7, wherein the uplink constraint information contains an informationelement for deriving start of the first timing.
 9. The first radiostation according to claim 7, wherein the first cell is a first type ofcells which is different from a second type of cells of the second cell.10. A communication control method in a radio terminal, comprising:supporting Dual Connectivity using a first cell served by a first radiostation and a second cell served by a second radio station; andreceiving uplink constraint information from the first radio station,wherein the uplink constraint information contains an informationelement for deriving transmission constraint in a case where the DualConnectivity using the first and second cells is performed, thetransmission constraint is a constraint related to uplink signaltransmission by the radio terminal over one or more subframe periods andindicates a first timing in which uplink signal transmission in thefirst cell is allowed, and a second timing in which uplink signaltransmission in the second cell is allowed is set so that the firsttiming and the second timing are not overlapped, and the communicationcontrol method further comprises: performing uplink transmission in thefirst cell and the second cell based on the uplink constraintinformation.
 11. A communication control method in a first radiostation, comprising: serving a first cell; supporting Dual Connectivityusing the first cell and a second cell served by a second radio station;sending first constraint information to the second radio station;receiving second constraint information from the second radio station;and transmitting uplink constraint information to the radio terminal,wherein the first constraint information is a constraint related to atleast one of uplink signal transmission by the radio terminal anddownlink signal transmission by the first radio station in the firstcell, the second constraint information is a constraint related to atleast one of uplink signal transmission by the radio terminal anddownlink signal transmission by the second radio station in the secondcell, the uplink constraint information contains an information elementfor deriving transmission constraint in a case where the DualConnectivity using the first and second cells is performed, thetransmission constraint is a constraint related to uplink signaltransmission by the radio terminal over one or more subframe periods andindicates a first timing in which uplink signal transmission in thefirst cell is allowed, the transmission constraint causes the radioterminal to set a second timing in which uplink signal transmission inthe second cell is allowed so that the first timing and the secondtiming are not overlapped.